I have lower back pain which causes my left leg to go numb right behind the kneecap.

Nerve Impingement: Your lower back pain is likely caused by either a compressed nerve between two vertebrae in your low back, or a compressed nerve between two spasmed muscles. The sciatic nerve must travel through both of these anatomic structures en route to the legs on either side of the body. In absence of more concerning signs, it's often helpful to address potential muscle spasm with heat and NSAIDs first.

Back pain and leg: You are describing a nerve root compression syndrome at your lumbar spine region. Sciatica is a word often used to describe a similar syndrome. Sciatica typically indicates pain down the side of the leg. Your syndrome though is similar. Numbness reflects nerve root compression as the nerve comes out of the spine. You require special Rx: Physical Therapy, Pain management, perhaps lumbar injections.

L4 or unrelated: L5 and S1 nerve root impingement produces discomfort that radiates below the knee all the way to the foot. L4 root can produce numbness down to the knee but usually not just behind the knee. Baker's cysts and other problems behing the knee joint can produce numbness behaind the knee but are unrelated to back pain. To sort this all out you will need to undergo an assessment.